1. Field of the Invention
The present invention relates to a soldering flux and a solder composition containing the same.
2. Description of the Related Art
In many conventional soldering fluxes, an activator comprised of organic acids or halogenated salts are added to rosins and denatured resins of rosins. In many cases, however, such a flux can remains as a residue on a printed-circuit board after completion of soldering operation and the residues cause corrosion of substrate materials, migration, and etc. Furthermore, when the printed-circuit board on which the residue remains is sealed with a sealing resin (e.g. a silicone gel and an epoxy resin), the residue inhibits the sealing resin from curing, affecting adhesion to the substrate, as well as electrical insulation. In order to remove the residues, cleansing off with alternatives for chlorofluorocarbons, organic solvents and etc. is carried out after completion of soldering operation. However, cleaning agents have been regulated due to environmental problems concerning the chlorofluorocarbons, VOC and etc.
An epoxy flux is one of fluxes which cause no corrosion, migration, or inhibition of the sealing resin from curing, even if the residual flux is not cleansed off. The epoxy flux comprises an epoxy resin as a main component, an activator such as carboxylic acid, amines, a thixo agent and the others. When articles of the printed board are mounted with creamy solder using the epoxy flux, the curing reaction of the epoxy resin with the carboxylic acid are designed to be simultaneous with an activation of the conductor surface by the carboxylic acid during reflow soldering, and also designed to be completed around the time when the articles are adhered following melting the solder. After the reflow soldering, the cured epoxy resin remains as a residue. Contrary to the rosin flux which has been commonly used, such a cured epoxy resin does not inhibit the sealing resin from adhering to the printed board even if the sealing is carried out without cleansing off after soldering the article, and moreover, it has superior electrical insulation (see Japanese Patent Application Laid-Open No. 2000-216300).
However, there are some problems as described below.
The Conventional epoxy fluxes have been used for lead-containing solders. The initiation temperature of curing reaction between the epoxy resin and the activator in the conventional epoxy fluxes is about 150xc2x0 C., slightly lower than the melting point of the lead containing solder (e.g. 63Sn-37Pb solder has a melting point of 183xc2x0 C.), and the curing reaction is completed around the time when the lead containing solder is melted and the article is adhered.
However, if such a conventional epoxy flux is used for lead-free solders (i.e. the solder containing no lead component) having higher melting points than the lead containing solders such as Snxe2x80x94Ag containing solders having melting points of about 220xc2x0 C., much of the carboxylic acid (the activator) is consumed in the curing reaction with the epoxy resin prior to melting of the solder and the activating power of the carboxylic acid can not be maintained, resulting in lowered fluidity of the flux causing lowered wettability of the solder, and etc.
Accordingly, it is an object of the present invention to provide a soldering flux applicable to lead-free solder as well as a lead-free solder composition containing said soldering flux and the lead-free solder, wherein even if soldering is carried out with the lead-free solder having higher melting point than lead containing solder (e.g. lead-free solder having a melting point of 190-240xc2x0 C.), the activation by the flux can be maintained, the solder has high wettability, and the sealing resin is not inhibited from curing even without cleansing off the residual flux.
The foregoing purpose of the present invention can be effectively achieved by providing a soldering flux containing an epoxy resin and an organic carboxylic acid, wherein said epoxy resin and said organic carboxylic acid are compounded at a ratio of 1.0 equivalent of epoxy group in said epoxy resin to 0.8-2.0 equivalent of carboxyl group in said organic carboxylic acid, and total amount of said epoxy resin and said organic carboxylic acid contained in said soldering flux is 70% or more by weight based on said total weight of said soldering flux.
Here, a temperature at the top of an exothermic peak in flux-curing reaction in which said epoxy resin and said organic carboxylic acid are polymerized may be 180-250xc2x0 C.
Here, an initiation temperature of flux-curing reaction in which said epoxy resin and said organic carboxylic acid are polymerized may be 180-230xc2x0 C.
Here, said epoxy resin and said organic carboxylic acid are compounded at a ratio of 1.0 equivalent of epoxy group in said epoxy resin to 0.8-1.1 equivalent of carboxyl group in said organic carboxylic acid.
Here, said total amount of said epoxy resin and said organic carboxylic acid contained in said soldering flux is 80% or more by weight based on said total weight of said soldering flux.
Here, alcohols may be further contained in an amount of 30% or less by weight based on said total weight of said soldering flux.
Here, alcohols may be further contained in an amount of 20% or less by weight based on said total weight of said soldering flux.
Here, said epoxy resin may be selected from the group consisting of bisphenol-A type epoxy resins, bisphenol-F type epoxy resins, novolac type epoxy resins, alicyclic epoxy resins and mixtures thereof.
Here, said bisphenol-A type epoxy resins is the bisphenol-A type epoxy resins having about 160 to 250 g/eq of epoxy equivalent.
Here, said carboxylic acid may be selected from the group consisting of saturated aliphatic dicarboxylic acids, unsaturated aliphatic dicarboxylic acids, alicyclic dicarboxylic acids, aromatic dicarboxylic acids, amino group containing carboxylic acids, hydroxyl group containing carboxylic acids, heterocyclic dicarboxylic acids and mixtures thereof.
Here, said carboxylic acid may be selected from the group consisting of succinic acid, glutaric acid, adipic acid, azelaic acid, dodecanoic diacid, itaconic acid, citraconic acid, mesaconic acid, cyclobutanedicarboxylic acid, cyclohexanedicarboxylic acid, cyclohexenedicarboxylic acid, cyclopentanetetracarboxylic acid, dimethylglutaric acid, methyladipic acid, glutamic acid, aspartic acid, ethylene-diaminetetraacetic acid, citric acid, malic acid, tartaric acid, pyridinedicarboxylic acid, pyrazinedicarboxylic acid, diglycolic acid, phenylenediacetic acid, catecholdiacetic acid, hydroquinonediacetic acid, thiopropionic acid, thiodibutyl acid, dithioglycolic acid and mixtures thereof.
Here, said carboxylic acid may be selected from the group consisting of cyclohexenedicarboxylic acid, dimethylglutaric acid, glutamic acid, phthalic acid and mixtures thereof.
Here, said alcohols may be selected from the group consisting of monoalcohols, polyhydric alcohols, and mixture thereof.
Here, said polyhydric alcohol may be selected from the group consisting of ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, octene glycol, polyethylene glycol, propanediol, glycerin and mixtures thereof.
Here, said monoalcohol may be selected from the group consisting of methyl alcohol, ethyl alcohol, propyl alcohol, butyl alcohol, isobutyl alcohol, amyl alcohol, isoamyl alcohol, octanol, allyl alcohol, cyclohexanol and mixtures thereof.
According to another aspect of the present invention, there is provided a solder composition containing the aforementioned soldering flux and a lead-free solder having a melting point of 190-240xc2x0 C.
Here, said lead-free solder may be a Sn containing lead-free solder having a melting point of 190-240xc2x0 C.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof.